Research On The Cocatalysts For The Copolymerization Of CO₂ And Epoxides

Alternating copolymerization of carbon dioxide with epoxides to make aliphatic polycarbonates (APC) is one of the significant achievements for the chemical fixation of carbon dioxide. These polycarbonates not only exhibit attractive properties, but also display great potential as biodegradable polymeric materials, which have brought them extensive concerns as CO₂-based materials. Great efforts have been dedicated directly toward the development of catalysts and mechanism of this copolymerization during the past forty years. By analyzing all of these investigations mentioned herein, such a noticeable conclusion can be drawn definitely, that is, activities and selectivity of the copolymerization catalyzed by tetradentate ligand complexes can be remarkably enhanced in the presence of cocatalysts, most of which are N-containing compounds.In this dissertation, a series of imidazolium-based ionic liquids, quaternary ammonium salts as well as pyridinium have been introduced as cocatalyst for the copolymerization of CO₂ with epoxides catalyzed by salenCr ^(III)Cl, which will greatly enrich the catalytic system in term of their tunable properties. This pronounced regularity between microstructure and macroscopic properties of ionic liquids may enlighten us to devise novel and efficient types of task-specific cocatalysts to improve the copolymerization.A series of experiments were elaborately conducted using salen Cr^(III) Cl and different imidazolium salts as dual catalytic system to explore the influence of different anions with fixed cations firstly. It seems that anions with higher nucleophilicity and better leaving ability, such as Cl^- and Br^- are benefit for improving the activities and increasing the percentage of carbonate linkages than BF₄^- and PF₆^-analogues. The utilities of [C₄mim]Cl and [C₄mim]Br as cocatalyst exhibited a turnover frequency (TOF) of 130.1h^-1 and 108.8h^-1, which is higher as compared with [C₄mim]PF₆ (TOF 54.7h^-1) and [C₄mim]BF₄ (TOF 91.2h^-1). Additionally, theseimidazolium-based ionic liquids performed slightly better than TV-methylimidazole (N-Melm) except [C₄mim]PF₆.An inspiring correlation between the alkyl chain length of imidazolium cations and copolymerization results is also observable in this studied system. As illustrated, the rates of copolymer production were gradually enhanced and catalytic activities of (salen)Cr^(III)C1 were over 2-fold upon increasing the alkyl chain length from n-butyl to n-dodecyl (TOF 108.8h^-1 versus TOF 242.5h^-1). It indicates that imidazolium salts with longer alkyl chain length can dramatically result in enhanced activities, better selectivity, and lower polydispersity index, which justify the tunable properties of ionic liquids and close connection between structures and activities.The molar ratio of monomer to catalyst as well as cocatalyst to catalyst was also studied here. It showed that higher molar ratio of monomer to catalyst increased the catalytic efficiency of catalyst. Additionally, the observed results also showed a pronounced dependence on the cocatalyst loading when keeping the concentration of catalyst constant. As indicated in the experimental results, the activities were substantially enhanced upon initially increasing the concentration of imidazolium salts like [C6mim]Br. However, there was a dramatic decrease in the activities of catalyst as well as an increase in polyether linkages at a higher cocatalyst loading (more than 30 equiv). Reasonable explanations have been presented in this dissertation combining the proposed mechanism.The influence of temperature on the copolymerization is also obvious. At a lower temperature, the catalyst was deactivated with carbonate linkages around 62.5%. At an increased temperature of 80℃, the activities of catalyst were greatly enhanced with carbonate linkages above 97%. When further increasing the temperature, copolymerization was readily facilitated towards alternating polymers. However, cyclic carbonates were additionally produced at elevated temperatures.Several quaternary ammonium salts were introduced as cocatalysts based on the previous achievements. The results further verify the pronounced regularity between structural modification of alkyl chain length and the corresponding coupling results. As illustrated, catalytic activities of (salen)Cr^(III) Cl were near to 9-fold uponincreasing the alkyl chain length from methyl to n- dodecyl (TOF 14. 1h^-1 versus TOF 121.2h^-1). The carbonate linkages were above 99% for trimethyldodecylammonium bromide, as compared with tetramethylammonium chloride 58%.The effects of pyridinium as cocatalyst on the copolymerization were studied at last. The catalytic activities were slightly enhanced on increasing alkyl chain length from pyridine to [C₁₆Py]Br (TOF 20.3h^-1 versus TOF 47.6h^-1), which was in accordance with the previous results.